This diagram is only a small portion (that we will cover in this post) of all the supported KML elements. If you are interested, please see the whole tree here.
In the tree, you can consider each element as a class in OOP, and an element is an extension of its parent. For example, <Placemark> is a special kind of Feature: a <Placemark> contains all of the attributes that a Feature has, and it adds some attributes that are specific to the <Placemark>.
Further, as you notice, there are two kinds of elements in the tree: